Abstract 3181: Expression changes of the chromatin modifier PBRM1 in human renal cell carcinomas in relation to histopathological features

Abstract

Abstract Recent next-generation sequencing studies of clear cell renal cell carcinoma (ccRCC) have identified point mutations in chromatin-modifying genes, among them PBRM1 which is now the second most frequently mutated gene in RCCs (in up to 41% ccRCCs cases) and belongs to the m1 TCGA (The Cancer Genome Atlas) subset. The occasional presence of PBRM1 mutations in the absence of VHL mutations indicates, that PBRM1 might be another driver tumor suppressor gene in ccRCCs. We were interested in examining PBRM1 expression at mRNA and protein levels and correlated these findings to histopathological features of RCC patients. Relative gene expression (RGE) data were obtained by Q-PCR from matched tumor and adjacent normal fresh frozen ccRCC tissues from 57 patients who underwent radical nephrectomy at our clinic. Papillary (pRCC) as well as chromophobic cases (chRCC) were added to analyze the expression of splice variants of PBRM1. Comparative Western blot analyses as well as semi-quantitative immuno-histochemical staining (IHC) for PBRM1 and VHL were performed on all ccRCC cases. In 78.9% of cases (45/57) PBRM1 mRNA was downregulated at least 1.5-fold, 7% (4/57) showed PBRM1 upregulation, and 14% (8/57) displayed no obvious expression changes between tumor and corresponding tumor tissue. Interestingly, 21 of 45 ccRCCs tumor tissues expressed splice variant 4 more abundantly when compared to normal tissue, whereas the normal tissue preferentially expressed splice variant 1 that includes an additional exon. Remarkably, this differential expression picture is completely reversed in pRCC and chRCC cases. The majority of 57 ccRCCs displayed weak nuclear PBRM1 staining (52.6%), whereas 31.6% showed moderate and 15.8% strong staining. However, we were not able to demonstrate a significant correlation of IHC expression levels, neither to tumor staging nor to Fuhrman grading. The observed high frequency of decreased expression of the chromatin-remodeling gene PBRM1 on mRNA (78.9%) and protein levels (52.6%), respectively, as wells as a high mutation rate (about 15% in our data set) indicate a substantial role of PBRM1 in the tumorigenesis of ccRCCs. Preferences in the expression of different PBRM1 splice variants warrant further investigation with regard to renal cell carcinoma development. Citation Format: Hans Krause, Anica Högner, Burkhard Jandrig, Ergin Kilic, Carsten Kempkensteffen. Expression changes of the chromatin modifier PBRM1 in human renal cell carcinomas in relation to histopathological features. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3181.